"I've been crying for hours, I feel like I let my fans down 2nite," she said via Twitter. "An hour before the show, I was feeling dizzy and having trouble breathing.

"Paramedics came to take care of me, and told me my heart-rate was irregular-- a result of exhaustion and dehydration," she added. "Can't apologize enough for how sorry I am. I could hear my fans cheering from my dressing room, I begged everyone to let me go onstage."

She said she was so "so devastated" because she would never "cancel a show just based on discomfort." Said the singer, 23: "I have performed with the flu, a cold, strep throat."

In this case, though, "everyone was concerned I'd be in danger during the 2hr show since I had passed out earlier," she wrote.

Before signing off, she wrote, "I hope you can forgive me. I love my little monsters more than anything, you are everything to me."

She said she will give a "make-up the performance" Jan. 26 at Purdue University.